A metal ring is placed vertically in the gravity field. A metal rod of length L and mass m is pivotally fixed in the center of the ring and touches it with the other end. A uniform magnetic field of induction B is perpendicular to the ring plane. By what law is it necessary to change the current in the rod so that the rod rotates evenly with an angular velocity ω, if at the initial moment the rod was in the upper position? Ignore the friction.